Partial solar eclipse

Denoting the radius of the Sun as $R$, and the radius of the Moon as $r$, we have that $R=2r$.
Therefore, the area of the Moon is $\pi r^{2}$, while the area of the Sun is $\pi R^{2}=\pi (2r)^{2}=\pi\times 4r^{2}=4\pi r^{2}$.

That is, the area of the Sun is 4 times bigger !
So one quarter of the Sun is blocked at this moment of the eclipse.